Mine had the exact same problem. The heater core is plugged. I used a garden hose with a pressure nozzle and flushed the heater hoses both directions, got some junk out but didn't make a difference. I took it to a local radiator shop and they used something with water and air and flushed the system and it works again.... not 100% but I get about 103 degree air from it. cost me eighty bucks plus coolant. the only other option is replace the heater core.